Electron microscopy studies of high coercive melt-spun Sm2+δFe15Ga2C2 permanent magnets

Abstract

The influence of over-stoichiometric Sm contents on the magnetic properties of melt-spun Ga stabilized Sm2Fe17 carbides has been investigated. For optimum heat treated Sm2+δFe15Ga2C2 ribbon flakes an increase of the room temperature coercivity could be observed from μ0HC=1.5 T for stoichiometric samples to μ0HC=2.2 T for δ=0.15. This effect is very interesting because of the improved temperature stability. At 500 K a maximum energy density of (BH)max=31.4 kJ/m3 for δ=0.09 can be observed. It is assumed that a Sm-rich nonmagnetic intergranular phase decouples the grains magnetically and is consequently the origin of the high coercivities of these samples. Electron microscopy studies of these melt-spun Sm2+δFe15Ga2C2 samples with different Sm contents have been performed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A broad grain size distribution (50–250 nm) is observed in the SEM, but there is no difference among samples with different Sm contents. TEM studies show no change in grain shape with variation of the δ value. High resolution TEM (HRTEM) studies indicate the existence of a very thin intergranular phase for samples with an excess of Sm which was proved using high resolution energy dispersive x-ray analysis (EDX) studies with a scanning TEM. These investigations show a significant increase of the Sm content at the grain boundaries for these highly coercive samples.